Johnson, pictured last month, was admitted to hospital after his coronavirus symptoms persisted. (PA)

A spokesman for Johnson said: “Since Sunday evening, the prime minister has been under the care of doctors at St Thomas’ Hospital, in London, after being admitted with persistent symptoms of coronavirus.

“The PM is receiving excellent care, and thanks all NHS staff for their hard work and dedication.”

Foreign secretary Dominic Raab (right) said during Monday's coronavirus press briefing he last spoke to the prime minister on Saturday. (PA)

The prime minister was last seen when he tweeted a video message update about his condition on Friday, when he still had a temperature but was not in hospital. (Twitter)

It was reported on Monday night that Johnson, 55, experienced breathing difficulties earlier in the day and was given oxygen.

When it became clear that he had to be moved to intensive care, the prime minister told an aide that Raab should deputise for him, according to The Times.

Mr Raab’s first job as stand-in PM on Tuesday will be to lead the Government’s emergency daily coronavirus “war Cabinet” meeting with senior colleagues and the UK’s chief medical officer, Professor Chris Whitty, and chief scientific adviser, Sir Patrick Vallance.

The prime minister tweeted this image of his virtual cabinet meeting held via Zoom while he was self isolating at the end of March. (PA)

The foreign secretary said the prime minister is in “safe hands” and has been receiving “excellent care”, adding the government was focused on “making sure we can defeat coronavirus”, although he admitted that the pair last spoke on Saturday.

The prime minister himself tweeted on Monday lunchtime that he was ‘in good spirits and keeping in touch with my team’.

Last night, on the advice of my doctor, I went into hospital for some routine tests as I’m still experiencing coronavirus symptoms. I’m in good spirits and keeping in touch with my team, as we work together to fight this virus and keep everyone safe.

Johnson’s fiancee, Carrie Symonds, previously said she had been suffering COVID-19 symptoms, but is “on the mend”. Ms Symonds, 32, who is expecting the couple’s first baby in early summer, said she was not tested for the virus.

Labour leader Sir Keir Starmer tweeted: “Terribly sad news. All the country’s thoughts are with the Prime Minister and his family during this incredibly difficult time.”

Chancellor Rishi Sunak tweeted: “My thoughts tonight are with @BorisJohnson and @carriesymonds. I know he’ll be getting the best care possible and will come out of this even stronger.”

Mr Johnson’s predecessor Theresa May tweeted: “My thoughts and prayers are with @BorisJohnson and his family as he continues to receive treatment in hospital. This horrific virus does not discriminate. Anyone can get it. Anyone can spread it. Please #StayHomeSaveLives.”

Police outside St Thomas' Hospital in central London, where the prime minister was moved to intensive care on Monday evening. (PA)

Other world leaders offered messages of support.

Donald Trump said Johnson was in a “very serious” condition to have been admitted to intensive care and offered prayers for his recovery.

At a White House press briefing on Monday, Mr Trump said: “We’re very saddened to hear that he was taken into intensive care this afternoon and Americans are all praying for his recovery.

French president Emmanuel Macron was the first major leader to send his wishes, tweeting: “All my support to Boris Johnson, his family and the British people at this difficult time. I wish him well.”

Canadian Prime Minister Justin Trudeau tweeted: “Sending my best wishes to Prime Minister @BorisJohnson for a full and speedy recovery. My thoughts are with you and your family right now. Hope to see you back at Number 10 soon.”

Timeline

March 27

Johnson revealed he had tested positive for COVID-19. Downing Street said he noticed mild symptoms the previous afternoon and received the test results at midnight.

In a video message on Twitter, he said: “I’m working from home and self-isolating and that’s entirely the right thing to do.

“But, be in no doubt that I can continue thanks to the wizardry of modern technology to communicate with all my top team to lead the national fightback against coronavirus.

It follows a period when the House of Commons had continued to sit, with Cabinet meetings and daily press briefings held in person throughout the first weeks of March.

March 31

Mr Johnson tweets an image of the first ever Cabinet meeting conducted entirely online. Some question whether the PM should be continuing to work while suffering from the virus.

April 2

The PM comes out of self-isolation for a brief appearance at the door of No 11 Downing Street, where he lives, to join in the mass clapping for key workers on the frontlines of the coronavirus pandemic.

He told those gathered outside: “I am not allowed out really, I am just standing here.”

April 3

Mr Johnson is seen in a video urging people to stay home and not to break social distancing rules as the weather warmed up, even if they were going “a bit stir crazy”.

Downing Street said Mr Johnson has been admitted to a London NHS hospital for tests as a “precautionary step” as his coronavirus symptoms persist.

April 6

Mr Johnson tweets: “Last night, on the advice of my doctor, I went into hospital for some routine tests as I’m still experiencing coronavirus symptoms. I’m in good spirits and keeping in touch with my team, as we work together to fight this virus and keep everyone safe.

“I’d like to say thank you to all the brilliant NHS staff taking care of me and others in this difficult time. You are the best of Britain.”

Just hours later, Downing Street said the Prime Minister’s condition had worsened and, on the advice of his medical team, he had been moved to the hospital’s intensive care unit.